1. Field of Invention
Actuator systems and methods that control printing systems by adjusting tone reproduction curve targets using real-time feedback control.
2. Description of Related Art
In copying or printing systems such as a Xerographic copier, laser printer or inkjet printer, a common technique for monitoring the quality of prints is to artificially create a test patch of a predetermined desired density. The actual density of the printing material, toner or ink for example, in the test patch can then be optically measured to determine the effectiveness of the printing process to place the correct quantity of material on the printed sheet.
With laser printers, a charge retentive surface or photoreceptor is used to form an electrostatic latent image that causes toner particles to adhere to areas on the surface that are charged in a particular way. An optical device, often referred to as a densitometer, may be used for determining the density of toner on the test patch (that can assume halftone levels from 0 to 100%) along the path of the photoreceptor and directly downstream of the development unit. The printing system may perform a process to periodically create test patches at the desired halftone levels at predetermined locations on the photoreceptor by deliberately actuating the exposure system.
The electrostatic latent test patch is then moved past a developer unit. Toner particles within the developer unit are caused to adhere to the test patch electrostatically. The developed test patch is moved past the densitometer disposed along the path of the photoreceptor and the specular reflectance and or diffuse reflectance of the test patch is measured. The density of toner on the patch varies in relationship to both the specular reflectance and diffuse reflectance of the test patch.
Xerographic test patches that are used to measure the deposition of toner on the photoreceptor, and thereby regulate the deposition of toner onto paper and control the tone reproduction curve (TRC) are traditionally printed in inter-document zone regions of photoreceptor belts or drums. Generally, each patch is a small square that is printed at a predefined halftone level. This practice enables the sensor to infer the TRC. The number of patches to monitor and regulate can range from 1 to the full number of halftone levels the system is capable of addressing.
Many Xerographic printing system process control systems adjust physical actuators such as developer bias, charge level and raster output scanner (ROS) intensity to maintain the TRC as measured by an in-line optical sensor. In the example presented here the controls maintain the TRC at three control points, though more or less control points can be used. Currently, there are insufficient actuators and insufficient latitude to control the entire TRC to the desired accuracy across the expected set of disturbances anticipated in a customer environment. The variation can cause objectionable color changes, especially in overlay colors that are printed using more than one of the printer primary colors.
Accordingly, because of the difficulty in monitoring and controlling the toner development process, various approaches have been devised.
U.S. Pat. No. 5,963,244 to Mestha et al. discloses sensing the TRC at discrete intervals and doing a least squares fit to project an entire TRC. The tone reproduction curve is recreated by providing a look-up table for reconstruction of the TRC. The look-up table incorporates a co-variance matrix of elements containing end-tone reproduction samples. The matrix multiplier responds to sensed developed patch samples and to the look-up table to reproduce a complete tone reproduction curve. A controller reacts to the reproduced tone reproduction curve to adjust machine quality.
U.S. Pat. No. 5,749,020 to Mestha et al. discloses TRC variations using a set of orthogonal basis functions. The basis functions are derived by decomposing sample tone reproduction curves to provide a predicted tone reproduction curve. The predicted tone reproduction curve is melded with a discrete number of tone reproduction samples to produce a reconstructed TRC for machine control.
U.S. Pat. No. 6,035,152 to Craig et al. discloses a method for measuring tone reproduction curves. A setup calibration TRC is generated based on preset representative halftone patches. A test pattern including a plurality of halftone patches is marked in the inter-document zone of the imaging surface. A relative reflection of each of the halftone patches is entered into a matrix and the matrix is correlated to a plurality of print quality actuators. A representative TRC is generated based on the matrix results. A feedback signal is produced by comparing the representative TRC to the setup calibration tone curve and each of the print quality actuators is adjusted independently to adjust printing machine operation for print quality correction.
U.S. Pat. No. 5,777,656 to Henderson discloses using lookup tables to adjust a measured TRC to match a target TRC. The method of maintaining tone reproduction for printing includes the steps of marking representative halftone targets on an imageable surface with toner sensing an amount of toner on each of the representative halftone targets, generating a representative TRC based on the sensed amount of toner on the representative halftone targets, producing a feedback signal generated by comparing a representative TRC to a setup calibration tone curve and adjusting pixel data of each pixel of the final halftone image to compensate for deviation between the representative TRC and the setup calibration tone curve.
U.S. Pat. No. 5,649,073 to Knox et al. discloses a method and apparatus for calibrating gray reproduction schemes for use in a printer. The calibration system includes a test pattern stored in a memory and providing a plurality of samples of combinations of printed spots printable on a media by the printer. A gray measuring device is included to derive a gray measurement of the samples of printed spots. A calibration processor correlates the gray measurements with a combination of spots having a particular spatial relationship and derives parameters describing the printer response to the combination. The calibration processor generates from the derived parameters at least one non-linear gray image correction function then stores the generated gray image function calibration in a calibration memory. A means is provided to apply the gray image correction stored in the calibration memory to calibrate a printer using a halftone pattern.
U.S. Pat. No. 5,612,902 to Stokes discloses a method and system for automatically characterizing a color printer. A relatively few number of test samples are printed and measured to create an analytic model which characterizes a printer. The analytical model is used in turn to generate a multi-dimensional look-up table that can then be used at one time to compensate image input and create a desired visual characteristic in the printed image.
Because of the potential near-degeneracy, e.g., ill-conditioned behavior, of the TRC response to actuator adjustments, Xerographic conditions arise under which holding fixed test patch targets can require driving the xerographic actuators to their limiting values. As discussed above, deadbanding has been introduced to mitigate these problems. However, while deadbanding can reduce the likelihood of forced excursions, deadbanding treats all actuator levels equally and does not adjust the actuators to preferable values while satisfying the constraint to keep the TRC within the specified dead band. Undesirable actuator levels may continue to be used because there is no restoring function to recenter the undesirable actuator level once within the deadband. Undesirable actuator levels are those that result in image quality defects that are not embodied by the TRC (even though the TRC is maintained close to target). Current systems can also exhibit increased color variability even under Xerographic conditions that would normally permit tight control to the TRC patch targets.